Screen for paper stock



y 3, 1956 A. G. SANDISON SCREEN FOR PAPER STOCK 2 Sheets-Sheet 1 Filed NOV. 8 1951 N N RN .IZTEE'DE .EL EXQNUEEG. fiazvzzrsm A. G- SANDISQN SCREEN FOR PAPER STOCK July 3, 1956 2 Sheets-Sheet 2 Filed Nov. 8, 195] METER ALEmmEE G. fiazszmsm nited States Patent Cfice 2,752,828 Patented July 3, 1956 SREEN Ffilirl EAPER S'iUiCK Alexander G. Sandison, Brantford, Qrntario, Canada, assignor to Waterous Limited, llrantfor-d, @Etiflllt), Canada, a corporation of Canada Application November- 8, 1951, Serial No. 255,355

Claims priority, application Canada September it, 1951 9 Claims. (Cl. QZ -Sd) This invention relates to apparatus for screening suspensions of solids in liquid, and in particular for screening paper stock.

When paper stock is forced under pressure through a screen plate having small perforations, a mat of fibre forms very rapidly on the plate and prevents the passage even of small fibres which would otherwise be able to pass through the perforations. For this reason screening apparatus for paper stock must be provided with means for breaking up the mat as soon as it forms.

A method that has been employed to break up the mat is to provide means for producing pulsations of momentary suction or reverse pressure across the screen at rapid intervals. Reverse flow occurs through the screen during each momentary interval of reversed pressure and this reverse flow breaks up the mat, permitting the screening action to proceed as soon as the direction of pressure changes back again to a driving pressure. The output per unit area of the screen plate depends on the intensity of the driving pressure forcing the stock through the screen, on the relative intensity and duration of the reversed pressure, and on the frequency of the pulsations. In particular it is found that high capacity per unit area of plate can be obtained only if a fairly high driving pres sure is employed.

One method of producing pulsations of the type described is disclosed in United States Patent 1,670,473, issued May 22, 1928, to S. Milne. Milnes circular straining drum is rotatable and is mounted in a vat. Disposed within the drum is a shaft on which is mounted a number of rings by means of spokes or arms. On the outer circumference of these rings are provided impellers consisting of shallow blades, which on rotation of the drum impart an area of increased pressure at their leading edges and an area of decreased pressure at their trailing edges. Thus a pulsating motion is given to the paper stock adjacent the screen.

As these pulsations of increased and decreased pressure are produced by the action of the blades cutting through the stock they would not occur to any sufficient degree it the stock were allowed to be carried round by the drag of the blades at a speed approaching that of the blades themselves. To prevent this an air space is provided below the level of the top of the drum. Such an air space, however, can only be maintained if the screen is operated at relatively low static pressure and relatively low blade velocities, so that low output of screened stock per unit area results.

In another type of pulp screen, known in the pulp industry as the centrifugal class of screen, the stock is impelled at high velocity, by means of a rotating element provided with paddles, around the inside of a screening surface which is a surface of revolution, usually a cylinder, and a mat formation is prevented by the scouring action which the stock exercises by reason of its high velocity and turbulence. Such screens consume relatively high power owing to the considerable kinetic energy which must be imparted to the stock.

In contradistinction to the prior apparatus, there is provided, according to the invention, the combination of streamline vanes immersed in the suspension and mounted for movement in a path adjacent to the screen plate, means for moving the vanes at a speed sufficient to produce suction in the space between each of the vanes and the plate, and a plurality of baffles for restraining the suspension from acquiring a velocity approaching that of the vanes in the direction of movement of the vanes.

A suitable type of streamlined shape for the vanes is that of an efficient airplane strut of fineness ratio about 3.5 measured parallel to the direction of movement of the vanes. As is well known, the drag of such a strut is only about six per cent of that of a round section of equal frontal area, and less than four per cent of that of a square section of equal frontal area, so that the vanes are enabled to cut through the stock and are accompanied by lateral areas of low pressure or suction, analagous to the area of suction on the upper surface of the wing of an airplane, and these areas of suction moving rapidly across the screening surface cause momentary intervals of reverse flow through the screen, thus breaking up the mat. As the vanes sweep around the screen so does the momentary reverse flow.

The baflles prevent the unscreened stock from acquir ing a velocity approaching that of the streamline vanes in the direction of movement of the vanes, such as would reduce the suction on the outer surface of the streamline vanes. The result is a screen which may be operated under relatively high pressure with relatively high vane speeds, leading to much higher output per unit area of screen surface. The action is more eifective than the scouring action used in centrifugal screens, and for equal peripherial velocities of the rotor, permits higher mean static pressures across the screening surface and results in much higher output per square foot of screening surface with much lower power consumption per unit volume of stock screened than is possible with the known types of machine referred to.

Provision is also made to render the screening apparatus at least partially self-regulating in regard to the consistency of the tailings or suspensions of oversize fibres rejected by the screen.

The invention will be more fully understood by reference to the accompanying drawings wherein:

Figure 1 represents a longitudinal sectional view of the apparatus and,

Figure 2 is a transverse section taken along the line ll-lI of Figure 1.

Referring to the drawings, unscreened stock is introduced by the inlet 1 into the space 2 enclosed by a cylindrical screen plate 3 which is supported by a frame 4. A shaft 5 coaxial to screen plate 3 is mounted on bearings 45 and carries a wheel 7 to which is secured vanes 3 located close to the screen plate 3. T he wheel 7 is formed on each side with radia vanes 9 and has inner holes it and outer holes 11 (Figure 2), through its web 12. On the shaft 5 is also mounted a labyrinth seal 13 and a flinger 1d.

The vanes 8 are of streamline section for the greater part of their length, but at their ends 15 remote from the stock inlet are formed to a square or other non-streamline section. The vane ends 15 lie within an annular chamber in connecting space 2 to a vortex chamber 1? which connects near its inner portion to an outletlti. An inlet 19 connects to a chamber 20 having holes 2i lead ing to the space 2, and also holes 22 leading to space 23 which is enclosed by stationary discs 24 and 255 and surrounds the wheel 7. Discs 24 and 25, and additional discs 26 are provided with radial vanes 27 and have holes 28. The screen plate 3 is encased in a housing 29 provided with an overflow dam 30 and outlet 3i for the accepted stock.

In operation, unscreened stock is admitted at a suitable static pressure through inlet 1, and the shaft 5 with wheel 7 and vanes 8 are rotated by power at an appropriate speed. The general effect of the static pressure is to promote outward flow of stock through the screen plate 3, but without the action of the vanes 8 a mat of fibre would rapidly form on the screen plate and stop the screening action. The passage of the vanes S, at suitable speed, produces local suction between the outer surface of each vane and the screen plate and causes a momentary back flow through the screen plate which breaks up the mat and permits effective screening action to resume under the action of the static pressure as soon as the vane has passed.

It will he understood that the production of effective suction on the back of the vanes depends on the relative velocity of vanes and stock, and that if the stock is permitted to rotate at a speed approaching that of the vanes the suction effect will be small and ineffective. Too rapid rotation of the stock is prevented by the vanes 27, which act as baffles to change any tangential velocity of the stock to inward velocity, the stock thus tending to flow inward at the vanes 27 and then to be forced outward again without substantial tangential velocity in the space between the vanes of adjacent discs. It will also be understood that the use of a streamline form for vanes 8 is essential to efiicient operation. If, for example, the vanes were of square section the resistance to their passage through the stock would be many times greater, and this would not only put up power consumption but would also reduce the effectiveness of the action as it would then be very difficult to prevent the stock rotating at a speed approaching that of the blades.

If the stock contains a considerable proportion of oversized fibres or impurities which will not pass through the screen perforations the stock will tend to thicken as it passes along the screen. Clear water, or in practice more frequently white water, which is the filtrate from thickeners or other filtering apparatus, is therefore admitted through the inlet 19, chamber 20, and holes 21 to dilute the stock to a lower consistency. A fraction of the white water also passes through the holes 22 to the space 23 surrounding the wheel 7. The water in this space is rotated by the vanes 9 and therefore acquires centrifugal head, so that a centrifugal seal is formed preventing stock from flowing into the space 23 and around the labyrinth 13. End thrust on the wheel '7 is prevented by the holes 11 which balance the water pressure near the periphery of the seal, and the holes It) which balance the air pressure near the centre of the seal. The labyrinth 13 serves to prevent any large outflow of stock in the event of stoppage of rotation rendering the centrifugal seal ineffective, out it is not quite tight and permits some passage of air. Thus any air trapped between discs 24 and 25 can work out through holes 28 formed in these discs and the clearance of the labyrinth 13.

The extensions 15 of vanes 8, being non-streamlined in section, will impart considerable circumferential velocity or swirl to the tailings passing through the annular space 16, and this swirl will persist in the vortex chamher 17, producing centrifugal back pressure against the flow of the tailings from space 16 to outlet 18. It will be understood that the persistance of this vortex will depend on the amount of stock flowing into it at high circumferential velocity, and also on the frictional elfect in the vortex chamber 17 in reducing the velocity of swirl, so that a balanced condition is obtained, which, for a given static head in the space 2 and a given consistency of tailings, will hold the rate of outflow substantially constant. If, however, the consistency of the tailings increases, the frictional effect will increase and the vortex velocity for a given flow will decrease so that it will require a greater flow to restore balance between the centrifugal back pressure and the static pressure in space 2. Thus an increase in the consistency of the tailings will also result in an increase in discharge, and the general action is such as will tend to maintain the consistency of the tailings fairly constant, as is desired for efiicicnt operation.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Screening apparatus comprising a cylindrical screen plate, means for admitting a suspension under pressure to the lr ide of said screen plate, a rotor mounted co-axially to said plate, streamline vanes mounted on said rotor and located to traverse a path adjacent to said plate on rotation of said rotor, means for rotating said rotor so as to move said vanes at a speed sutficient to produce suction in the spaces between each of said vanes and said screen, a plurality of hafi'les comprising substantially radial vanes restraining said suspension from acquiring velocity in the direction of movement of said vanes approaching the velocity of said streamline vanes, said baffles being located radially inwardly of the path travelled by said streamline vanes, a housing having an outlet for accepted suspension in connection with one side of said screen plate and an outlet for tailings in connection with the other side of said screen plate.

2. Screening apparatus comprising a cylindrical screen plate, means for admitting a suspension under pressure to the inside of said screen plate, a rotor mounted coaxially to said plate, streamline vanes mounted on said rotor and located to traverse a path adjacent to said plate on rotation of said rotor, means for rotating said rotor so as to move said vanes at a speed suiiicient to produce suction in the spaces between each of said vanes, a plurality of baffles restraining said suspension from acquiring velocity in the direction of movement of said vanes approaching the velocity of said streamline vanes, said baffies comprising fixed vanes located radially inwardly of the path travelled by said streamline vanes, a housing having an outlet for accepted suspension in connection with one side of said screen plate and an outlet for tailings in connection with the other side of said screen plate.

3. Screening apparatus comprising a cylindrical screen plate, means for admitting a suspension under pressure to the inside of said screen plate, a rotor mounted co axially to said plate, streamline vanes mounted on said rotor and located to traverse a path adjacent to said plate on rotation of said rotor, means for rotating said rotor so as to move said vanes at a speed sufficient to produce suction in the spaces between each of said vanes and said screen, a plurality of bafiles comprising substantially radial vanes restraining said suspension from acquiring velocity in the direction of movement of said vanes approaching the velocity of said streamline vanes, 21 housing having an outlet for accepting suspension in connection with one side of said screen plate, an outlet for railings in connection with the other side of said screen plate, a vortex chamber at one end of said screen plate substantially co-axial thereto, a tailings outlet connected to the radially inner portion of said vortex chamber, means connecting the outer portion of said vortex chamher with the inside of said screen plate for the passage of tailings to the radially outer portion of said vortex chamher, and means adapted to impart circumferential velocity to tailings in said vortex chamber to promote a centrifugal back pressure in said vortex chamber to oppose the pressure within said screen plate and regulate the discharge of tailings.

4. Screening apparatus as claimed in claim 3 comprising an annular chamber in substantial alignment with said streamline vanes and communicating with both the inside of said screen plate and outer portion of said vortex chamber.

5. Screening apparatus as claimed in claim 4 in which the means for imparting circumferential velocity to tailings in said vortex chamber comprises means projecting from the adjacent ends of said streamline vanes into said annular chamber.

6. Screening apparatus as claimed in claim 5 in which said means projecting into said annular chamber comprises non-streamline extensions of said streamline vanes.

7. Screening apparatus as claimed in claim 3 in which said baffles comprise fixed vanes located radially inwardly of the path travelled by said streamline vanes.

8. Screening apparatus comprising a screen plate, means for admitting a suspension under pressure to one side of said screen plate, streamline vanes immersed in said suspension and mounted for movement in a path adjacent to said plate, means for moving said vanes at a speed sufficient to produce suction in the space between each of said vanes and said plate, a plurality of baffles comprising substantially radial vanes restraining said suspension from acquiring a velocity in the direction of movement of said vanes approaching the velocities of said vanes, a housing having an outlet for accepted suspension in connection with one side of said screen plate and an outlet for tailings in connection with the other side of said screen plate.

9. Screening apparatus comprising a cylindrical screen plate, means for admitting a suspension under pressure to the inside of said screen plate, a rotor mounted coaxially to said plate, streamline vanes mounted on said rotor and located to traverse a path adjacent to said plate on rotation of said rotor, means for rotating said rotor so as to move said vanes at a speed sufficient to produce suction in the spaces between each of said vanes and said screen, and a plurality of baffies comprising substantially radial vanes restraining said suspension from acquiring velocity in the direction of movement of said vanes approaching the velocity of said streamline vanes, the housing having an outlet for accepted suspension in connection with one side of said screen plate and an outlet for railings in connection with the other side of said screen plate.

References Cited in the file of this patent UNITED STATES PATENTS 1,163,275 Spangenberg Dec. 7, 1915 1,670,473 Milne May 22, 1928 1,742,592 Hang Jan. 7, 1930 1,921,750 Heinrich Aug. 7, 1933 1,932,663 Haug Oct. 31, 1933 1,953,237 Klotz Apr. 3, 1934 

1. SCREENING APPARATUS COMPRISING A CYLINDRICAL SCREEN PLATE, MEANS FOR ADMITTING A SUSPENSION UNDER PRESSURE TO THE INSIDE OF SAID SCREEN PLATE, A ROTOR MOUNTED CO-AXIALLY TO SAID PLATE, STREAMLINE VANES MOUNTED ON SAID ROTOR AND LOCATED TO TRAVERSE A PATH ADJACENT TO SAID PLATE ON ROTA TION OF SAID ROTOR, MEANS FOR ROTATING SAID ROTOR SO AS TO MOVE SAID VANES AT A SPEED SUFFICIENT TO PRODUCE SUCTION IN THE SPACES BETWEEN EACH OF SAID VANES AND SAID SCREEN A PLURALITY OF BAFFLES COMPRISING SUBSTANTIALLY RADIAL VANES RESTRAINING SAID SUSPENSION FROM ACQUIRING VELOCITY IN THE DIRECTION OF MOVEMENT OF SAID VANES APPROACHING THE VELOCITY OF SAID STREAMLINE VANES, SAID BAFFLES BEING LOCATED RADIALLY INWARDLY OF THE PATH TRAVELLED BY SAID STREAMLINE VANES, A HOUSING HAVING AN OUTLET FOR ACCEPTED SUSPENSION IN CONNECTION WITH ONE SIDE OF SAID SCREEN PLATE AND AN OUTLET FOR TAILINGS IN CONNECTION WITH THE OTHER SIDE OF SAID SCREEN PLATE. 